Brain functions are executed by intricately coordinated networks of neurons, whose modes of operation are highly sensitive to a constellation of neuromodulators. More specifically, neuromodulators such as dopamine, norepinephrine, serotonin, and acetylcholine exert dramatic control over global brain processes such as arousal, attention, emotion, or cognitive perception. Altered neuromodulator signaling has been linked to neurological and psychiatric disorders such as Parkinson's disease, schizophrenia, depression and addiction. Similarly, opioid neuropeptides play important roles in the modulation of cognition and behavior. While the anatomical structures that produce neuromodulatory signals are well known, little is known about the spatial and temporal evolution of these signals in the innervated brain regions. This is because current measurement techniques, such as microdialysis or cyclic voltammetry, lack the spatial or temporal resolution (and often the molecular specificity) to resolve respective signals. This technical challenge has been a long-standing barrier to our understanding of how neuromodulation alters neural circuit function in order to influence behavior. To address this challenge, this project will develop, validate, and disseminate novel genetically encoded fluorescent proteins for large-scale optical measurement of monoamine neuromodulator and opioid neuropeptide signaling in behaving animals, by bringing together a multi-disciplinary team of investigators with unique and complementary expertise. These sensor proteins have the potential to revolutionize neuroscience in a way similar to genetically encoded indicators for calcium, glutamate, and voltage, which are now in widespread use. Combined with calcium and voltage imaging, neuromodulator sensors will reveal how these systems impinge on cellular and circuit function. In particular, proposed sensors will enable minimally invasive, high-resolution, long-term, and direct measurement of neuromodulator and neuropeptide signaling at synaptic, cellular, and system levels. Sensors for neuromodulatory signaling have remained elusive for a long time. Our team recently developed a first generation of genetically encoded indicators for serotonin (5-HT), norepinephrine (NE), and dopamine (DA) that can report nano- to micromolar concentration changes with high spatial and temporal resolution. Building on this work, the following specific aims are proposed: 1) Optimize and diversify genetically encoded sensors for the monoamines using computational modeling, directed evolution and high-throughput screening; 2) Develop and optimize genetically encoded sensors for opiate neuropeptides using novel protein scaffolds; and 3) Systematically validate the novel neuromodulator and neuropeptide sensors in acute brain slices and behaving animals. Together, this work will provide the neuroscience community with a wide range of well-characterized multi-color indicators for probing the functional role of neuromodulators and neuropeptides in regulating neural circuit function and behavior in both health and disease.